Yes, that works, but most people don't use that extra level of directory. They would just use /etc/portage/package.use/a as a regular file. There's no real need for a file type either because the only thing that goes into package.use is package-specific USE flags; no harm done, though.

This is all explained in the Working with Gentoo section of the Handbook. Time for some light reading?

- John_________________I can confirm that I have received between 0 and 999 National Security Letters.